The present invention relates generally to integrated circuit (IC) designs, and more particularly to an improved image sensor cell with a high dynamic range.
CMOS image sensors have offered significant advantages of cost and size over traditional sensor structures. Placing a semiconductor integrated circuit (IC) chip as an image sensor in the image plane of a camera greatly simplifies both still and video image recording and provides for flexible design of portable cameras. The CMOS image sensor is widely accepted because: 1) it operates at low voltages; 2) has low power consumption; 3) offers random access to the image data; 4) is compatible with CMOS logic technology; and 5) allows realization of an integrated single-chip camera. Typically, light from an image impinging on each active pixel sensor is sensed and translated into data, which may include a plurality of transistors, a photodiode and a current generator. For example, a three-transistor active pixel sensor is typically composed of a reset transistor, a source follower transistor, and a row selector transistor, with a photodiode, and a current generator.
An image produced by a camera ideally should be as similar as possible to the same image seen directly by the human eye. The human eye is less sensitive to changes in illumination in bright environments than in dark environments. In fact, the human eye responds to a wide range of illumination without hardly being saturated, meaning that the human eye is no longer sensitive to a change of illumination intensity. However, this is not how the conventional active pixel sensor operates. The conventional active pixel sensor has a linear response over its initial light level of illumination, and then abruptly saturates when the light level reaches a certain point. This operational feature is often referred to as the machine mode. When the saturation occurs, a bright light level will flood the photodiode of the sensor and drive it to a full current conduction. The abrupt saturation limits the dynamic range of the sensor. It is desirable that the senor captures an image with a quality as close as possible to the human eye does. Since the human eye hardly saturates, the image captured by the sensor with the limited dynamic range would not be very close to that observed by a human eye.
Therefore, there is a need for an image sensor that more nearly emulates the light response of the human eye.